Method semiconductor device fabrication with improved epitaxial source/drain proximity control

ABSTRACT

A semiconductor device includes a substrate, a first fin extending from the substrate, a first gate structure over the substrate and engaging the first fin, and a first epitaxial feature partially embedded in the first fin and raised above a top surface of the first fin. The semiconductor device further includes a second fin extending from the substrate, a second gate structure over the substrate and engaging the second fin, and a second epitaxial feature partially embedded in the second fin and raised above a top surface of the second fin. A first depth of the first epitaxial feature embedded into the first fin is smaller than a second depth of the second epitaxial feature embedded into the second fin.

PRIORITY

This is a divisional application of U.S. patent application Ser. No. 15/934,017, filed on Mar. 23, 2018, which claims priority to U.S. Provisional Pat. App. Ser. No. 62/591,961, filed on Nov. 29, 2017, the entire disclosure of which is herein incorporated by reference.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs.

For example, as the device geometry shrinks, coupling capacitance between source/drain features and nearby gates has increased. In some instances, the increased coupling capacitance lowers down the operating frequency of the transistors. This has become more evident in FinFET devices than in planar devices and has affected transistors for radio frequency (RF) devices (“RF transistors”) more adversely than those for logic devices (“logic transistors”). How to reduce the coupling capacitance for RF transistors (particularly RF FinFET transistors) while maintaining high overall transistor density in an IC is an object of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 illustrates cross-sectional views of a semiconductor structure, in portion, constructed according to an embodiment of the present disclosure.

FIGS. 2A and 2B show a block diagram of a method of forming a semiconductor device, according to various aspects of the present disclosure.

FIGS. 3A, 3B, 3C, 3D, 3E, and 3F illustrate perspective views of a semiconductor device, in portion, at various fabrication stages according to the method of FIGS. 2A and 2B, in accordance with some embodiments.

FIGS. 3G and 3H illustrate cross-sectional views of a semiconductor device, in portion, at some fabrication stages according to the method of FIGS. 2A and 2B, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

The present disclosure is generally related to methods for semiconductor device fabrication, and more particularly to methods of forming field effect transistors (FETs) having raised epitaxial S/D features in RF devices (or circuits) and logic devices (or circuits). For example, RF devices may perform power amplification, splitting, and/or combining; and logic devices may perform input/output (I/O) functions and logic functions including AND, OR, NOR, and inverters, as well as other functions. Generally, RF devices operate at a frequency about ten times higher than logic devices. For example, RF devices may operate at a frequency in a range from 10 GHz to 500 GHz, such as from 20 GHz to 500 GHz, while logic devices may operate at a frequency below 20 GHz. Further, RF devices and logic devices may be in separate regions of an IC or may be mixed in a common region of an IC. Design objectives for transistors in RF devices (“RF transistors”) and those in logic devices (“logic transistors”) may be different. For RF transistors, a lower coupling capacitance between source/chain (S/D) features and nearby gates is highly desired so as to improve the operating frequency of the RF transistors.

Referring to FIG. 1 , shown therein are cross-sectional views of a semiconductor device 100, in portion, constructed according to the present disclosure. The semiconductor device 100 includes a first transistor 101 a and a second transistor 101 b. The transistors 101 a and 101 b are FinFET in the shown embodiment. Alternatively, they may be planar transistors or other types of three-dimensional transistors. Particularly, the transistor 101 a is a logic transistor (i.e., a transistor performing a logic function of an IC), and the transistor 101 b is an RF transistor (i.e., a transistor performing an RF function of an IC).

The logic transistor 101 a includes a semiconductor fin 104 a, epitaxial S/D features 142 a at least partially embedded in the semiconductor fin 104 a, a gate structure 128 a engaging the semiconductor fin 104 a between the epitaxial S/D features 142 a, gate spacers 109 on sidewalls of the gate structure 128 a, and interlayer dielectric (ILD) layer 144 over the semiconductor fin 104 a, the S/D features 142 a, and surrounding the gate spacers 109 and the gate structure 128 a.

The RF transistor 101 b includes a semiconductor fin 104 b, epitaxial S/D features 142 b at least partially embedded in the semiconductor fin 104 b, a gate structure 128 b engaging the semiconductor fin 104 b between the epitaxial S/D features 142 b, gate spacers 109 on sidewalls of the gate structure 128 b, and the ILD layer 144 over the semiconductor fin 104 b, the S/D features 142 b, and surrounding the gate spacers 109 and the gate structure 128 b.

The epitaxial S/D features 142 a are distanced from the gate structure 128 a by a first distance (or proximity) X₁ along the lengthwise direction of the semiconductor fin 104 a. The epitaxial S/D features 142 b are distanced from the gate structure 128 b by a second distance (or proximity) X₂ along the lengthwise direction of the semiconductor fin 104 b. In the present embodiment, the lengthwise directions of the semiconductor fins 104 a and 104 b are the same (both along the X direction). In alternative embodiments, the lengthwise directions of the semiconductor fins 104 a and 104 b may be different. In other words, the semiconductors fins 104 a and 104 b may be oriented along the same or different directions in various embodiments. In the present embodiment, the distance X₁ is smaller than the distance X₂. In an embodiment, a ratio between X₁ and X₂ is in a range from 1:1.2 to 1:3, such as from 1:2 to 1:3. The specific ratios are designed to provide the following benefits: a smaller distance X₁ allows the epitaxial S/D features 142 a to apply greater stress to the channel of the logic transistor 101 a and to increase carrier mobility thereof, and a greater distance X₂ reduces the coupling capacitance C₂ between the epitaxial S/D features 142 b and the gate structure 128 b thereby improving frequency response of the RF transistor 101 b. For the logic transistor 101 a, the coupling capacitance C₁ between the epitaxial S/D features 142 a and the gate structure 128 a may be greater than the coupling capacitance C₂, but is within an acceptable range for logic devices that operate at a lower frequency than the RF devices. The above range of ratio (X1:X2 is about from 1:1.2 to 1:3) is selected to balance out the two competing factors: coupling capacitance and carrier mobility, for both the logic transistor 101 a and the RF transistor 101 b in the same integrated circuit. If the ratio is outside this range, then either the logic transistor 101 a or the RF transistor 101 b may suffer from degraded performance, for example, resulting in slower operation speed. For example, if X1 is kept as is and X2 is too big, then the coupling capacitance in the RF transistor decreases but its carrier mobility decreases too, decreasing its operation speed. For example, if X2 is kept as is and X1 is too big, then the coupling capacitance in the logic transistor decreases but its carrier mobility decreases too, decreasing its operation speed.

Further, the epitaxial S/D features 142 a and 142 b extend into the fins 104 a and 104 b by depths D₁ and D₂, respectively. In an embodiment, the depth D₂ is configured to be greater than D₁. This effectively increases the width of the source/drain features, as well as the width of the channel region of the RF transistor 101 b, thereby increasing the saturation current of the RF transistor 101 b. With a higher saturation current, the RF transistor 101 b provides a higher oscillation frequency and voltage gain for the benefits of RF applications. Still further, a portion of each epitaxial S/D feature 142 a that extends above the fin 104 a has a volume V₁, and a portion of each epitaxial S/D feature 142 b that extends above the fin 104 b has a volume V₂. In an embodiment, the volume V₂ is configured to be smaller than the volume V₁. Having a smaller volume V₂ above the fin 104 b reduces the coupling capacitance C₂ between the gate 128 b and the epitaxial features 142 b, thereby improving frequency response of the RF transistor 101 b.

In various embodiments, the epitaxial features 142 b may be doped with a higher concentration of a dopant than the epitaxial features 142 a. For example, when both the transistors 101 a and 101 b are NMOSFETs, the epitaxial features 142 b may be doped with a higher concentration of phosphorus (P) or other suitable dopants than the epitaxial features 142 a. For another example, when both the transistors 101 a and 101 b are PMOSFETs, the epitaxial features 142 b may be doped with a higher concentration of boron (B) or other suitable dopants than the epitaxial features 142 a. Having a higher dopant concentration increases the carrier mobility in the RF transistor 101 b. In some embodiments, both the epitaxial features 142 a and 142 b include silicon germanium (e.g., both the transistors 101 a and 101 b are PMOSFETs), but the germanium concentration (relative to silicon) is higher in the epitaxial features 142 b than in the epitaxial features 142 a. For example, the epitaxial features 142 a includes Si_(1-x)Ge_(x) alloy and the epitaxial features 142 a includes Si_(1-y)Ge_(y) alloy, where x and y represent ratios in atomic numbers and x is smaller than y. This increases the stress to the channel region and increases carrier mobility in the RF transistor 101 b.

Even though two transistors are shown in this embodiment, the device 100 may include any number of transistors configured in any number of device regions. Further, the device 100 may include other types of transistors in addition to FinFETs, such as planar transistors, gate all-around transistors, and nano-wire transistors. Still further, the device 100 may include passive components such as resistors, capacitors, and inductors, and active components such as p-type FETs (PFETs), n-type FETs (NFETs), metal-oxide semiconductor field effect transistors (MOSFET), complementary metal-oxide semiconductor (CMOS) transistors, bipolar transistors, high voltage transistors, high frequency transistors, static random access memory (SRAM), other memory cells, and combinations thereof. More details of the device 100 will be discussed below in conjunction with FIGS. 2A-3H which illustrate manufacturing steps for fabricating the device 100.

Referring to FIGS. 2A and 2B, shown therein is a method 200 of forming the semiconductor device 100 according to various aspects of the present disclosure. The method 200 is an example, and is not intended to limit the present disclosure beyond what is explicitly recited in the claims. Additional operations can be provided before, during, and after the method 200, and some operations described can be replaced, eliminated, or moved around for additional embodiments of the method. The method 200 is described below in conjunction with FIGS. 3A-3H which show perspective and cross-sectional views of the semiconductor device 100 according to various aspects of the present disclosure.

Referring to FIG. 2A, at operation 202, the method 200 receives or is provided with a workpiece of the semiconductor device (or semiconductor structure) 100. For the convenience of discussion, the workpiece is also referred to as the semiconductor structure 100. Referring to FIG. 3A, the semiconductor structure 100 includes a substrate 102 having a first region 102 a and a second region 102 b. Various components of the logic transistor 101 a are fabricated in or over the first region 102 a, and various components of the RF transistor 101 b are fabricated in or over the second region 102 b. The device 100 may include p-type logic FinFETs, n-type logic FinFETs, p-type RF FinFETs, and n-type RF FinFETs. For the sake of simplicity, the transistors 101 a and 101 b are assumed to be of the same type in the following discussion. In an embodiment, the transistors 101 a and 101 b are both p-type FinFETs. In another embodiment, the transistors 101 a and 101 b are both n-type FinFETs.

Over the first region 102 a, the semiconductor structure 100 includes various semiconductor fins (or “fins”) 104 a (two shown) that are disposed side-by-side, and a gate structure 108 a engaging the fins 104 a over top and sidewalls thereof. Over the second region 102 b, the semiconductor structure 100 includes various fins 104 b (two shown) that are disposed side-by-side, and a gate structure 108 b engaging the fins 104 b over top and sidewalls thereof. In various embodiments, each of the transistors 101 a and 101 b may include any number of semiconductor fins, such as single fin, dual fin, triple fin, and so on. Further, the transistors 101 a and 101 b may include different number of fins.

The semiconductor structure 100 further includes an isolation structure 106 over the substrate 102. The fins 104 a and 104 b protrude out of the substrate 102 and through the isolation structure 106. The semiconductor structure 100 further includes one or more dielectric layers (“dielectric spacers”) 109 disposed over top and sidewalls of the gate structures 108 a and 108 b and of the fins 104 a and 104 b. The various components of the semiconductor structure 100 are further described below.

The substrate 102 is a silicon substrate in the present embodiment. Alternatively, the substrate 102 may include another elementary semiconductor, such as germanium; a compound semiconductor including silicon carbide, gallium arsenide, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof. In yet another alternative, the substrate 102 is a semiconductor-on-insulator (SOI), such as a semiconductor substrate having a buried dielectric layer on which the fins 104 a and 104 b stand.

The semiconductor fins 104 a and 104 b may include one or more semiconductor materials such as silicon, germanium, or silicon germanium. In an embodiment, each of the semiconductor fins 104 a and 104 b may include multiple different semiconductor layers stacked one over the other. The semiconductor fins 104 a and 104 b may be fabricated using suitable processes including double-patterning or multi-patterning processes. Generally, double-patterning or multi-patterning processes combine photolithography and self-aligned processes, allowing patterns to be created that have, for example, pitches smaller than what is otherwise obtainable using a single, direct photolithography process. For example, in one embodiment, a sacrificial layer is formed over a substrate and patterned using a photolithography process. Spacers are formed alongside the patterned sacrificial layer using a self-aligned process. The sacrificial layer is then removed, and the remaining spacers, or mandrels, may then be used to pattern the semiconductor fins 104 a and 104 b by etching initial epitaxial semiconductor layers of the substrate 102. The etching process can include dry etching, wet etching, reactive ion etching (RIE), and/or other suitable processes.

The isolation structure 106 may include silicon oxide, silicon nitride, silicon oxynitride, fluoride-doped silicate glass (FSG), a low-k dielectric material, air gap, and/or other suitable insulating material. The isolation structure 106 may be a shallow-trench-isolation (STI), field oxide, LOCal Oxidation of Silicon (LOCOS), and/or other suitable structures. The isolation structure 106 may include a multi-layer structure, for example, having one or more thermal oxide liner layers. In an embodiment, the isolation structure 106 is formed by etching trenches in the substrate 102, e.g., as part of the fins 104 a and 104 b formation process. The trenches may then be filled with isolating material, followed by a chemical mechanical planarization (CMP) process and/or an etch-back process. The isolating material may be deposited by any suitable deposition techniques including chemical vapor deposition (CVD), plasma enhanced CVD (PECVD), and flowable CVD (FCVD).

Each of the gate structures 108 a and 108 b may include a gate dielectric layer, a gate electrode layer, and a hard mask layer, one over the other in that order. The gate structures 108 a and 108 b may include one or more additional layers. In an embodiment, the gate structures 108 a and 108 b are sacrificial gate structures, i.e., placeholders for final gate structures (or gate stacks). The gate dielectric layer may include a dielectric material such as silicon oxide, and may be formed by chemical oxidation, thermal oxidation, atomic layer deposition (ALD), CVD, and/or other suitable methods. The gate electrode layer may include polycrystalline silicon (polysilicon) or amorphous silicon, and may be formed by suitable deposition processes such as low-pressure CVD (LPCVD) and PECVD. The hard mask layer may include one or more layers of silicon nitride or silicon oxide, and may be formed by CVD, physical vapor deposition (PVD), or other suitable deposition techniques. In an embodiment, the various layers of the gate structure 108 a and 108 b are first deposited as blanket layers over the isolation structure 106 and the fins 104 a and 104 b. Then the blanket layers are patterned through a process including photolithography processes and etching processes thereby removing portions of the blanket layers and keeping the remaining portions as the gate structures 108 a and 108 b.

The dielectric spacers 109 may include one or more layers of dielectric materials, such as silicon oxide, silicon nitride, silicon oxynitride, silicon carbon nitride, a low-k nitride, or a combination thereof. For example, the dielectric spacers 109 may include a silicon nitride layer over a silicon oxide layer. In an embodiment, the dielectric spacers 109 have a thickness in a range from about 2 to about 10 nm. The dielectric spacers 109 may be deposited using CVD, ALD, or other suitable methods.

At operation 204, the method 200 (FIG. 2A) forms a first masking element 111 over the second region 102 b. Referring to FIG. 3B, in an embodiment, the first masking element 111 includes a photoresist patterned by a photolithography process. The photolithography process may include coating a photoresist (or resist) layer over the substrate 102; exposing the resist layer to a pattern, performing post-exposure baking; and developing the resist layer to form a patterned resist layer. In a further embodiment, the first masking element 111 includes a bottom antireflective coating (BARC) layer and a resist layer over the BARC layer. The resist layer is patterned using a photolithography process as discussed above, and the BARC layer is subsequently etched (e.g., by dry etching, wet etching, or other etching methods) using the patterned resist layer as an etch mask. The patterned BARC layer and resist layer become part of the first masking element 111. In an embodiment, the first masking element 111 covers every part of the semiconductor structure 100 except those regions for fabricating a particular type of logic devices, such as n-type logic devices or p-type logic devices, including the logic transistor 101 a.

At operation 206, the method 200 (FIG. 2A) performs a first etching process 124 to the semiconductor structure 100 while the first masking element 111 is in place (FIG. 3C). In the present embodiment, the first etching process 124 is applied to all n-type logic devices or all p-type logic devices, but not both. The first etching process 124 may include one or more steps. For example, a first step may be tuned to remove the dielectric spacers 109 from the top surfaces of the gate structures 108 a, the fins 104 a, and the isolation structure 106; and a second step may be tuned to recess the fins 104 a to form recesses 140 a. In an embodiment, the first etching process 124 includes a dry etching process that may implement an oxygen-containing gas, a fluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, NF₃, and/or C₂F₆), a chlorine-containing gas (e.g., Cl₂, CHCl₃, CCl₄, and/or BCl₃), a bromine-containing gas (e.g., HBr and/or CHBR₃), an iodine-containing gas, other suitable gases and/or plasmas, and/or combinations thereof. As a result of the operation 206, the dielectric spacers 109 are partially removed, with some portions remaining on the sidewalls of the gate structure 108 a, which become gate spacers 109. Also, in the present embodiment, some portions of the dielectric spacers 109 that are on lower sidewalls of the fins 104 a (see FIGS. 3B and 3C) remain and become fin sidewall (FSW) spacers 109. In some embodiments, the portions of the dielectric spacers 109 that are on sidewalls of the fins 104 a may be completely removed. In the present embodiment, the first etching process 124 is tuned (e.g., by adjusting etching chemical(s), duration, temperature, pressure, bias, etc.) such that the fins 104 a are recessed below the top surface of the isolation structure 106 along the Z direction and are recessed laterally along the X direction, thereby extending the recesses 140 a under the gate spacers 109. The first etching process 124 is designed to produce the recesses 140 a to have characteristics (e.g., depth, width, and shape) suitable for growing epitaxial S/D features 142 a as shown in FIG. 1 for the logic transistor 101 a. More aspects of the first etching process 124 and the recesses 140 a are further described after operation 214 (FIG. 2B).

At operation 208, the method 200 (FIG. 2A) removes the first masking element 111, for example, by resist stripping or ashing or other suitable methods. At operation 210, the method 200 (FIG. 2A) forms a second masking element 113 over the first region 102 a. Referring to FIG. 3D, in an embodiment, the second masking element 113 includes a photoresist patterned by a photolithography process. In a further embodiment, the second masking element 113 includes a BARC layer under the resist layer. The method of forming the second masking element 113 is similar to that of forming the first masking element 111. In an embodiment, the second masking element 113 covers every part of the semiconductor structure 100 except those regions for fabricating a particular type of RF devices, such as n-type RF devices or p-type RF devices, including the RF transistor 101 b. Particularly, the type of RF devices (either n-type or p-type) is the same as the type of logic devices (either n-type or p-type) that is exposed by the first masking element 111 (see the operation 204).

At operation 212, the method 200 (FIG. 2A) performs a second etching process 126 to the semiconductor structure 100 while the second masking element 113 is in place (FIG. 3E). In the present embodiment, the second etching process 126 is applied to all n-type RF devices or all p-type RF devices, but not both. The second etching process 126 may include one or more steps. For example, a first step may be tuned to remove the dielectric spacers 109 from the top surfaces of the gate structures 108 b, the fins 104 b, and the isolation structure 106; and a second step may be tuned to recess the fins 104 b to form recesses 140 b. In an embodiment, the second etching process 126 includes a dry etching process that may implement an oxygen-containing gas, a fluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, NF₃, and/or C₂F₆) a chlorine-containing gas (e.g., Cl₂, CHCl₃, CCl₄, and/or BCl₃), a bromine-containing gas (e.g., HBr and/or CHBR₃), an iodine-containing gas, other suitable gases and/or plasmas, and/or combinations thereof. As a result of the operation 212, the dielectric spacers 109 are partially removed, with some portions remaining on the sidewalls of the gate structure 108 b, which become gate spacers 109. Also, in the present embodiment, some portions of the dielectric spacers 109 that are on lower sidewalls of the fins 104 b (see FIGS. 3D and 3E) remain and become fin sidewall (FSW) spacers 109. In some embodiments, the portions of the dielectric spacers 109 that are on sidewalls of the fins 104 a may be completely removed.

In the present embodiment, the second etching process 126 is tuned to differ from the first etching process 124 (such as isotropy/anisotropy, etching duration, etching temperature, etching bias, etching power, etching pressure, a concentration of an etching chemical, or a ratio among etching chemicals) such that the recesses 140 b have different characteristics (e.g., depth, width, or shapes) than the recesses 140 a (see FIG. 3G). Particularly, the recesses 140 b extend under the gate spacers 109 for a shorter distance than the recesses 140 a do (E₂<E₁ and X2>X1 in FIG. 3G). In some embodiments, the recesses 140 b may be etched shallower than or about same depth as the recesses 140 a. In a particular embodiment, the recesses 140 b are etched deeper than the recesses 140 a (D₂>D₁ in FIG. 3G)) so that the epitaxial features 142 b may extend deeper into the fins 104 b than the epitaxial features 142 a extend into the fins 104 a (see FIG. 1 ). For example, deeper epitaxial features 142 b increase conducting area for the transistor fin, thereby increasing the drivability of the transistor 101 b. For example, D2 may be greater than D1 by 20% to 80%. In other words, D2:D1 may range from 1.2:1 to 1.8:1 in some embodiments. If the ratio is smaller than this range, then the benefits of increased drivability of the transistor 101 b diminishes. If the ratio is bigger than this range, which means D1 is too small, then the transistor 101 a may suffer from degraded performance. In various embodiments, the difference between the depths of the recesses 140 a and 140 b and the difference between the widths of the recesses 140 a and 140 b (therefore proximity to the gates 108 a and 108 b respectively) can be separately controlled by tuning the etching processes 124 and 126. For example, each of the etching processes 124 and 126 may utilize a physical etching (e.g., anisotropic etching using HBr or other suitable etchants) to control the difference in the recess depths D₁ and D₂. Also, each of the etching processes 124 and 126 may utilize a chemical etching (e.g., isotropic etching using an etchant having NF₃, F₂, Cl₂, or other suitable chemicals) to control the difference in the recess proximity X₁ and X₂.

Further, in the physical etching and chemical etching, some parameters such as etching duration, etching temperature, etching bias, etching power, etching pressure, concentration of etching chemicals, or a ratio among etching chemicals can be further tuned to create desired depth and proximity profiles in the recesses 140 a and 140 b. For example, in physical etching, using longer etching duration, higher etching temperature, higher etching bias, higher etching power, higher etching pressure, or a combination thereof results in deeper recesses. For example, in chemical etching, using longer etching duration, higher etching temperature, higher etching pressure, higher concentration of etching chemical, or a combination thereof results in wider recesses. In embodiments, the concentration of the etching chemical may be tuned by adjusting etching chemical gas flow rate(s) and/or the inert carrier gas flow rate(s) into the etching chamber. In another embodiment where each of the first and second etching processes 124 and 126 is performed with a mixture of etching chemical gases, the ratio of etching chemical gases in the mixture may be tuned differently for the two etching processes so that the etching rates of the two etching processes are different.

In some embodiments, the first and second etching processes 124 and 126 are tuned to differ in only one parameter that is one of: etching duration, etching bias, etching temperature, etching power, a concentration of an etching chemical, and a ratio among etching chemicals, while all other parameters are the same between the two etching processes. This may make it easier to control the difference between the recesses 140 a and 140 b. In alternative embodiments, the first and second etching processes 124 and 126 are tuned to differ in a combination of two or more parameters selected from, but not restricted to, etching duration, etching bias, etching temperature, etching power, a concentration of an etching chemical, and a ratio among etching chemicals. The first and second etching processes 124 and 126 may be performed in the same process chamber with different etching recipes. Alternatively, they may be performed in different etching chambers.

At operation 214, the method 200 (FIG. 2B) removes the second masking element 113 (FIG. 3F) by resist stripping or ashing or other suitable methods. In an embodiment, the operation 214 may perform a cleaning process to the recesses 140 a and 140 b to make them ready for epitaxial growth. FIG. 3G illustrate cross-sectional views of the semiconductor structure 100 at this fabrication stage along the A-A and B-B lines of FIG. 3F, which cut the fins 104 a and 104 b along the X direction. Referring to FIG. 3G, the recesses 140 a are etched into the S/D regions 110 a of the fins 104 a, and the recesses 140 b are etched into the S/D regions 110 b of the fins 104 b. The gate structure 108 a engages the channel regions 112 a of the fins 104 a. The gate structure 108 b engages the channel regions 112 b of the fins 104 b. The gate spacers 109 include a seal spacer 130 (e.g., silicon oxide) and a main spacer 132 (e.g., silicon nitride) in the present embodiment. Each of the recesses 140 a and 140 b may extend directly under the main spacer 132 but not the seal spacer, or may extend directly under both the main spacer 132 and the seal spacer 130.

The recesses 140 a have a depth D₁ (as measured from the top surface of the fin 104 a to the bottom surface of the recess 140 a), and the recesses 140 b have a depth D₂ (as measured from the top surface of the fin 104 b to the bottom surface of the recess 140 b). In some embodiments, D₁ is greater than or about equal to D₂. In the present embodiment, D₁ is smaller than D₂. Further, the distance X₁ between the recesses 140 a and the gate structure 108 a (as measured from the vertical extension of a side of the gate structure 108 a to the nearest surface of the recesses 140 a along the X direction) is smaller than the distance X₂ between the recesses 140 b and the gate structure 108 b (as measured from the vertical extension of a side of the gate structure 108 b to the nearest surface of the recesses 140 b along the X direction). In an embodiment, a ratio between X₁ and X₂ is in the range from 1:1.2 to 1:3, such as from 1:2 to 1:3. Still further, the recesses 140 a extend under the gate spacer 109 for a distance E₁ (as measured from the vertical extension of a side of the gate spacer 109 to the nearest surface of the recesses 140 a along the X direction), and the recesses 140 b extend under the gate spacer 109 for a distance E₂ (as measured from the vertical extension of a side of the gate spacer 109 to the nearest surface of the recesses 140 b along the X direction). In the present embodiment, E₁ is greater than E₂. The differences between the dimensions of the recesses 140 a and 140 b may be controlled by tuning the first and second etching processes 124 and 126 as discussed above. In the above embodiment, the recesses 140 a are etched before the recesses 140 b. Alternatively, the recesses 140 b may be etched before the recesses 140 a. In other words, the etching processes 124 and 126 may be performed in any order.

At operation 216, the method 200 (FIG. 2B) epitaxially grows a first semiconductor material 142 a in the recesses 140 a (FIG. 3H). At operation 218, the method 200 (FIG. 2B) epitaxially grows a second semiconductor material 142 b in the recesses 140 b (FIG. 3H). The semiconductor materials 142 a and 142 b are the epitaxial S/D features for the transistors 101 a and 101 b, respectively. In an embodiment, the operations 216 and 218 may be performed simultaneously by growing the same semiconductor material in the recesses 140 a and 140 b. This saves production time. In an alternative embodiment, the operations 216 and 218 may be performed separately. For example, the operation 216 may be performed after the operation 206 but before the operation 208, while the operation 218 may be performed after the operation 212 but before the operation 214. Performing the operations 216 and 218 separately allows different semiconductor materials to be epitaxially grown for the S/D features 142 a and 142 b for the purpose of separately tuning the performance of logic devices and RF devices. Of course, the semiconductor materials of the S/D features 142 a and 142 b can still be the same even if the operations 216 and 218 are performed separately. Due to the difference in the recesses 140 a and 140 b as discussed above, the S/D features 142 b are embedded deeper in the fin 104 b than the S/D features 142 a are embedded in the fin 104 a in the present embodiment (D₂>D₁). Further, the S/D features 142 a are closer to the respective gate structure 108 a (with a distance X₁) than the S/D features 142 b are relative to the respective gate structure 108 b (with a distance X₂). Still further, each of the S/D features 142 a and 142 b are grown out of the respective recesses 140 and 140 b and raised above the respective fins 104 a and 104 b. For example, each S/D feature 142 a has a portion 142 a-1 embedded in the fin 104 a and another portion 142 a-2 raised above the fin 104 a. Similarly, each S/D feature 142 b has a portion 142 b-1 embedded in the fin 104 b and another portion 142 b-2 raised above the fin 104 b. In an embodiment, the operations 216 and 218 are controlled such that the S/D portion 142 b-2 has a smaller volume than the S/D portion 142 a-2. Having a smaller volume in the raised portion 142 b-2 reduces the coupling capacitance between the S/D features 142 b and the gate 108 b. In some examples, the operations 216 and 218 may control the epitaxial growth parameters (e.g., growth time, growth precursors, growth pressure etc.) to create the different volumes in the raised portions 142 a and 142 b.

In an embodiment, the epitaxial growth processes in the operations 216 and 218 may be a low pressure chemical vapor deposition (LPCVD) process using a silicon-based precursor gas. Further, the epitaxial growth process may in-situ dope the grown S/D features 142 a and 142 b with an n-type dopant such as P, As, or combinations thereof for forming the S/D features for n-type FinFETs or with a p-type dopant such as B for forming the S/D features for p-type FinFETs. In an embodiment, the S/D features 142 b are doped with a higher concentration of the dopant than the S/D features 142 a so that the RF transistor 101 b provides higher carrier mobility. In some examples, for p-type FinFETs, the S/D features 142 a and 142 b may include silicon germanium, germanium or a combination. In an embodiment, both the S/D features 142 a and 142 b include silicon germanium, but the S/D features 142 b have a higher germanium to silicon ratio than the S/D features 142 a. For n-type FinFETs, the S/D features 142 a and 142 b may include silicon carbon, silicon, or a combination.

At operation 220, the method 200 (FIG. 2B) performs operations similar to the operations 204, 206, 208, 210, 212, 214, 216, and 218 to form S/D features in other regions of the substrate 102. In an embodiment, the transistors 101 a and 101 b are both n-type FinFETs. To further this embodiment, the operation 220 performs fabrications discussed above with reference to the operations 204-218 to form p-type FinFETs in the device structure 100. In another embodiment, the transistors 101 a and 101 b are both p-type FinFETs. To further this embodiment, the operation 220 performs fabrications discussed above with reference to the operations 204-218 to form n-type FinFETs in the device structure 100.

At operation 222, the method 200 (FIG. 2B) performs further steps to fabricate a final IC device including both RF and logic transistors. In an embodiment, the method 200 replaces the gate structure 108 a and 108 b with high-k metal gate stacks 128 a and 128 b, respectively. Referring to FIG. 1 , an inter-layer dielectric (ILD) layer 144 is formed over the substrate 102 (not shown in FIG. 1 , but see FIG. 3H) by a procedure, such as deposition and CMP. In an embodiment, the ILD layer 144 is formed by a flowable CVD (FCVD) process. The FCVD process includes depositing a flowable material (such as a liquid compound) on the substrate 102 to fill the gaps between the various structures (including the fins 104 a-b, the gate structures 108 a-b, and the gate spacers 109) and converting the flowable material to a solid material by a suitable technique, such as annealing in one example. Then, a CMP process is performed to the ILD layer 144 to expose the gate structures 108 a-b. Subsequently, the gate structures 108 a-b are removed by one or more selective etching processes thereby forming openings surrounded by the gate spacers 109 and the ILD 144. Subsequently, one or more material layers are deposited into the openings to form high-k metal gates 128 a and 128 b.

Each of the gate structures 128 a and 128 b may include an interfacial layer, a high-k dielectric layer, a work function metal layer, and a metal fill layer. The interfacial layer may include a dielectric material such as silicon oxide or silicon oxynitride, and may be formed by chemical oxidation, thermal oxidation, ALD, CVD, and/or other suitable dielectric. The high-k dielectric layer may include hafnium oxide (HfO₂), zirconium oxide (ZrO₂), lanthanum oxide (La₂O₃), titanium oxide (TiO₂), yttrium oxide (Y₂O₃), strontium titanate (SrTiO₃), other suitable metal-oxides, or combinations thereof. The dielectric layer may be formed by ALD and/or other suitable methods. The work function metal layer may be an n-type work function layer for n-type FinFETs or a p-type work function layer for p-type FinFETs, and may be deposited by CVD, PVD, and/or other suitable process. The p-type work function layer comprises a metal selected from but not limited to the group of titanium nitride (TiN), tantalum nitride (TaN), ruthenium (Ru), molybdenum (Mo), tungsten (W), platinum (Pt), or combinations thereof. The n-type work function layer comprises a metal selected from but not limited to the group of titanium (Ti), aluminum (Al), tantalum carbide (TaC), tantalum carbide nitride (TaCN), tantalum silicon nitride (TaSiN), or combinations thereof. The metal fill layer may include aluminum (Al), tungsten (W), or copper (Cu) and/or other suitable materials, and may be formed by CVD, PVD, plating, and/or other suitable processes. Further processes, such as contact and via formation, interconnect processing, etc., may be performed subsequently to complete the fabrication of the semiconductor structure 100.

Although not intended to be limiting, one or more embodiments of the present disclosure provide many benefits to a semiconductor device and the formation thereof. For example, embodiments of the present disclosure provide methods of forming raised epitaxial S/D features for both logic transistors and RF transistors in the same IC, and for controlling proximity of the epitaxial S/D features to nearby gate structures. Particularly, a ratio between the proximity in logic transistors to the proximity in RF transistors is disclosed, which improves frequency response for RF transistors while enhancing integration for logic transistors. Fabrication methods according to embodiments of the present disclosure can be readily integrated into existing manufacturing flow.

In one exemplary aspect, the present disclosure is directed to a method performed to a structure that includes a substrate with a first region configured for logic devices and a second region configured for radio frequency (RF) devices, a first fin over the first region, a first gate structure engaging the first fin, a second fin over the second region, a second gate structure engaging the second fin, and gate spacers over sidewalls of the first and second gate structures. The method includes forming a first masking element exposing portions of the first fin and covering the second region; performing a first etching process to the first fin, thereby resulting in a first recess proximate the first gate structure; removing the first masking element; forming a second masking element exposing portions of the second fin and covering the first region; and performing a second etching process to the second fin, resulting in a second recess proximate the second gate structure, wherein the first and second etching processes are tuned to differ in at least one parameter such that the first recess is shallower than the second recess and a first distance between the first recess and the first gate structure along the first fin lengthwise is smaller than a second distance between the second recess and the second gate structure along the second fin lengthwise.

In an embodiment, the method further includes epitaxially growing a first semiconductor material in the first recess; and epitaxially growing a second semiconductor material in the second recess. In a further embodiment, a volume of a portion of the second semiconductor material above the second fin is smaller than a volume of a portion of the first semiconductor material above the first fin. In another further embodiment, the second semiconductor material is doped with a higher concentration of a dopant than the first semiconductor material. In yet another embodiment, both the first semiconductor material and the second semiconductor material include silicon germanium, and the second semiconductor material has a higher germanium concentration than the first semiconductor material.

In an embodiment of the method, a ratio of the first distance to the second distance is in a range from 1:1.2 to 1:3. In a further embodiment, the ratio is in a range from 1:2 to 1:3. In another embodiment of the method, the at least one parameter includes etching duration, etching temperature, etching power, etching pressure, a concentration of an etching chemical, a ratio among etching chemicals, or a combination thereof. In another embodiment of the method, the first and second fins are both configured for forming p-type FinFETs or both configured for forming n-type FinFETs.

In an embodiment, the method further includes, before the forming of the second masking element, epitaxially growing a first semiconductor material in the first recess; and after the performing of the second etching process, epitaxially growing a second semiconductor material in the second recess. In another embodiment, the first and second etching processes are performed in a same process chamber.

In another exemplary aspect, the present disclosure is directed to a method. The method includes receiving a structure having a semiconductor substrate with a first region and a second region, a first fin over the first region, a first gate structure engaging the first fin, a second fin over the second region, a second gate structure engaging the second fin, and gate spacers over sidewalls of the first and second gate structures. The method further includes forming a first masking element covering the second region; performing a first etching process to the first fin while the first masking element is over the second region, thereby resulting in a first recess proximate the first gate structure, wherein the first recess extends under the gate spacers for a first distance; removing the first masking element; forming a second masking element covering the first region; and performing a second etching process to the second fin while the second masking element is over the first region, resulting in a second recess proximate the second gate structure. The second recess extends under the gate spacers for a second distance that is smaller than the first distance, wherein a difference between the first and second distances is due to at least one parameter in the first and second etching processes being different. The method further includes epitaxially growing a first semiconductor material in the first recess; and epitaxially growing the first semiconductor material in the second recess, wherein a volume of a first portion of the first semiconductor material above the first fin is larger than a volume of a second portion of the first semiconductor material above the second fin.

In an embodiment of the method, each of the first and the second etching processes includes an anisotropic etching process and an isotropic etching process. In another embodiment of the method, the first and the second etching processes are tuned to differ in one or more etching parameters including etching duration, etching temperature, etching bias, etching power, etching pressure, concentration of etching chemicals, or a ratio among etching chemicals.

In another embodiment of the method, the second recess is etched deeper than the first recess. In yet another embodiment of the method, the first semiconductor material in the second recess is doped with a higher concentration of a dopant than the first semiconductor material in the first recess.

In yet another exemplary aspect, the present disclosure is directed to a semiconductor device. The semiconductor device includes a substrate with a first region and a second region; first and second fins over the first and second regions, respectively; first and second gate structures engaging the first and second fins, respectively; gate spacers over sidewalls of the first and second gate structures; a first epitaxial feature at least partially embedded in the first fin and proximate the first gate structure; and a second epitaxial feature at least partially embedded in the second fin and proximate the second gate structure. A first distance between the first epitaxial feature and the first gate structure along the first fin lengthwise is smaller than a second distance between the second epitaxial feature and the second gate structure along the second fin lengthwise. A depth of the first epitaxial feature into the first fin is smaller than a depth of the second epitaxial feature into the second fin.

In an embodiment of the semiconductor device, a ratio between the first distance and the second distance is in a range from 1:1.2 to 1:3. In another embodiment of the semiconductor device, a portion of the first epitaxial feature above the first fin has a larger volume than a portion of the second epitaxial feature above the second fin. In yet another embodiment of the semiconductor device, the first and the second epitaxial features both include n-type doped silicon or both include p-type doped silicon germanium, and wherein the second epitaxial feature includes a higher concentration of dopants than the first epitaxial feature.

The foregoing outlines features of several embodiments so that those of ordinary skill in the art may better understand the aspects of the present disclosure. Those of ordinary skill in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those of ordinary skill in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A semiconductor device, comprising: a substrate having a first region configured for logic devices and a second region configured for radio frequency (RF) devices; a first fin extending from the first region of the substrate; a first gate structure over the substrate and engaging the first fin; a first gate spacer over sidewalls of the first gate structure; a first epitaxial feature partially embedded in the first fin and raised above a top surface of the first fin, wherein a portion of the first epitaxial feature is directly under the first gate spacer; a second fin extending from the second region of the substrate; a second gate structure over the substrate and engaging the second fin; a second gate spacer over sidewalls of the second gate structure; and a second epitaxial feature partially embedded in the second fin and raised above a top surface of the second fin, wherein a portion of the second epitaxial feature is directly under the second gate spacer, wherein a first depth of the first epitaxial feature embedded into the first fin is smaller than a second depth of the second epitaxial feature embedded into the second fin, wherein a first distance of the first epitaxial feature extending under the first gate spacer is larger than a second distance of the second epitaxial feature extending under the second gate spacer, and wherein the first and second fins include a same material composition.
 2. The semiconductor device of claim 1, wherein a ratio of the first depth to the second depth is in a range from 1:1.2 to 1:1.8, and wherein the first epitaxial feature is in direct contact with a sidewall and a bottom surface of the first gate spacer.
 3. The semiconductor device of claim 1, wherein a volume of a portion of the first epitaxial feature above the top surface of the first fin is larger than a volume of a portion of the second epitaxial feature above the top surface of the second fin.
 4. The semiconductor device of claim 1, wherein a first third distance between the first epitaxial feature and the first gate structure along the first fin lengthwise direction is smaller than a fourth distance between the second epitaxial feature and the second gate structure along the second fin lengthwise direction.
 5. The semiconductor device of claim 4, wherein a ratio of the third distance to the fourth distance is in a range from 1:1.2 to 1:3.
 6. The semiconductor device of claim 5, wherein the ratio is in a range from 1:2 to 1:3.
 7. The semiconductor device of claim 4, wherein the first fin lengthwise direction and the second fin lengthwise direction are the same.
 8. The semiconductor device of claim 4, wherein the first fin lengthwise direction and the second fin lengthwise direction are different.
 9. The semiconductor device of claim 1, wherein the second epitaxial feature is doped with a higher concentration of a dopant than the first epitaxial feature.
 10. The semiconductor device of claim 1, wherein both the first and second epitaxial features include germanium, and the second epitaxial feature has a higher germanium concentration than the first epitaxial feature.
 11. The semiconductor device of claim 10, wherein both the first and second epitaxial features include silicon germanium, and the second epitaxial feature has a higher germanium to silicon ratio than the first epitaxial feature.
 12. A semiconductor device, comprising: a substrate with a first region and a second region; first and second fins over the first and second regions, respectively; first and second gate structures engaging the first and second fins, respectively; first and second gate spacers over sidewalls of the first and second gate structures, respectively, wherein the first gate spacer is in direct contact with the first gate structure and the second gate spacer is in direct contact with the second gate structure; a first epitaxial feature at least partially embedded in the first fin and proximate the first gate structure; and a second epitaxial feature at least partially embedded in the second fin and proximate the second gate structure, wherein a first distance that is a shortest distance between the first epitaxial feature and the first gate structure along the first fin lengthwise direction is smaller than a second distance that is a shortest distance between the second epitaxial feature and the second gate structure along the second fin lengthwise direction, wherein a depth of the first epitaxial feature into the first fin is smaller than a depth of the second epitaxial feature into the second fin.
 13. The semiconductor device of claim 12, wherein a ratio between the first distance and the second distance is in a range from 1:1.2 to 1:3, and wherein the first epitaxial feature is in direct contact with a sidewall and a bottom surface of the first gate spacer.
 14. The semiconductor device of claim 12, wherein a portion of the first epitaxial feature above the first fin has a larger volume than a portion of the second epitaxial feature above the second fin.
 15. The semiconductor device of claim 12, wherein the first epitaxial feature extends under the first gate spacer for a third distance along the first fin lengthwise direction and the second epitaxial feature extends under the second gate spacer for a fourth distance along the second fin lengthwise direction, and the third distance is greater than the fourth distance, wherein a portion of the second epitaxial feature is directly under the second gate spacer.
 16. The semiconductor device of claim 12, wherein the first gate spacer includes a first main spacer and a first seal spacer, the second gate spacer includes a second main spacer and a second seal spacer, the first epitaxial feature fully extends under the first main spacer, and the second epitaxial feature partially extends under the second main spacer.
 17. The semiconductor device of claim 12, wherein the first and the second epitaxial features both include n-type doped silicon or both include p-type doped silicon germanium, and wherein the second epitaxial feature includes a higher concentration of dopants than the first epitaxial feature.
 18. A semiconductor device, comprising: a first transistor in a first region, wherein the first transistor includes: a first fin; a first gate structure engaging the first fin; a first gate spacer over sidewalls of the first gate structure; and a first epitaxial feature grown over the first fin, wherein a portion of the first epitaxial feature is directly under the first gate spacer; a second transistor in a second region, wherein the second transistor includes: a second fin; a second gate structure engaging the second fin; a second gate spacer over sidewalls of the second gate structure; and a second epitaxial feature grown over the second fin, wherein a portion of the second epitaxial feature is directly under the second gate spacer; and a dielectric layer over the first epitaxial feature in the first region and over the second epitaxial feature in the second region, wherein: a top surface of the first epitaxial feature is in direct contact with the dielectric layer, a top surface of the second epitaxial feature is in direct contact with the dielectric layer; the first gate spacer has a first sidewall in direct contact with the first gate structure and a second sidewall opposing the first sidewall and in direct contact with the dielectric layer, the second gate spacer has a third sidewall in direct contact with the second gate structure and a fourth sidewall opposing the third sidewall and in direct contact with the dielectric layer, a first distance of the first epitaxial feature extending under the first gate spacer is larger than a second distance of the second epitaxial feature extending under the second gate spacer, and a volume of a portion of the first epitaxial feature above the first fin is larger than a volume of a portion of the second epitaxial feature above the second fin.
 19. The semiconductor device of claim 18, wherein the first transistor is a logic transistor and the second transistor is a radio frequency (RF) transistor.
 20. The semiconductor device of claim 18, wherein a depth of the first epitaxial feature into the first fin is smaller than a depth of the second epitaxial feature into the second fin. 